This invention relates to the recovery of oil from subterranean reservoirs using carbon dioxide as a recovery agent and more particularly, to a method for controlling the mobility of carbon dioxide in such reservoirs.
In the recovery of oil from subterranean, oil-bearing formations, it is usually possible to recover only a limited proportion of the original oil present in the reservoir by the so-called primary recovery methods which utilize the natural formation pressure to produce the oil through suitable production wells. For this reason, a variety of supplementary recovery techniques have been employed, directed either to maintaining formation pressure or to improving the displacement of the oil from the porous rock matrix. Techniques of this kind have included formation pressurization, thermal recovery methods such as steam flooding and in situ combustion, water flooding and miscible flooding techniques.
In miscible flooding operations, a solvent is injected into the reservoir to form a single phase solution with the oil in place so that the oil can then be removed from the reservoir. This provides extremely effective displacement of the oil from the reservoir in the areas through which the solvent flows so that an extremely low residual saturation is obtained. The efficiency of this process derives from the fact that under the conditions of temperature and pressure prevailing in the reservoir, a two-phase system within the reservoir between the solvent and the reservoir oil is eliminated with the result that the retentive forces of capillarity and interfacial tension which are significant factors in reducing the recovery efficiency of oil in conventional flooding operations where the displacing agent and the reservoir oil exist as two separate phases are eliminated or substantially reduced.
Miscible recovery operations are normally carried out by a displacement procedure in which the solvent is injected into the reservoir to displace the oil from the reservoir and towards a production well from which the oil is produced. Because the solvent, typically a light hydrocarbon such as liquid petroleum gas (LPG) or a paraffin in the C.sub.2 to C.sub.6 range, may be quite expensive, it is often desirable to carry out the recovery by injecting a slug of the solvent, followed by a cheaper displacement liquid such as water.
Of the various enhanced oil recovery processes so far used or proposed, flooding by carbon dioxide is considered to be of substantial promise. In the CO.sub.2 flooding technique, a slug of carbon dioxide is injected into the formation to mobilize the oil and permit it to be displaced towards a production well at an offset distance from the injection well. Carbon dioxide, however, is considered a miscible-type flooding agent because under supercritical conditions, usually high pressure, carbon dioxide acts as a solvent and in certain reservoir situations, has a great advantage over more common fluids as a displacement agent. Even under conditions where the carbon dioxide is not wholly effective as a solvent for the oil, recovery may be improved by taking advantage of the solubility of carbon dioxide in the oil, causing a viscosity reduction and a swelling of the oil, which leads to increased recovery. These effects have been utilized at pressures much lower than the miscibility pressures for carbon dioxide and oil. Processes using carbon dioxide as a recovery agent are described in U.S. Pat. Nos. 3,811,501; 3,811,502 and 4,410,043.
The principal advantage of dense phase or supercritical carbon dioxide as a displacement fluid is the demonstrably high microscopic efficiency, close to 100%, with which it can displace crude oil from porous matrices. One problem which arises, however, is that carbon dioxide is much less viscous than oil or water and the result of this is that the injected fluid does not displace the oil uniformly. Instead, the carbon dioxide moves faster in some regions and directions than in others and "viscous fingers" are formed through which most of the injected fluids flow. Some of these fingers may arrive prematurely at the production well, lowering the effectiveness of both the injected carbon dioxide and of the production pumping capacity.
Although only hydrocarbons of fairly low molecular weight are miscible in all proportions with liquid or dense phase, supercritical carbon dioxide, high displacement efficiencies may be attained even with oils containing higher molecular weight components; i.e. "black" oils because even though these oils are not first contact miscible with carbon dioxide and contain significant quantities of high molecular weight heavy ends that have very low solubility in carbon dioxide, the dynamic processes of extraction and flow during the early stages of displacement by carbon dioxide at high pressures causes a transition region to develop between the crude oil and the carbon dioxide containing dissolved lower molecular weight components and this zone having a composition gradient varying from the crude oil to the carbon dioxide, is fully miscible in the fluids bounding the zone. After development of this miscible zone, it can remain intact and displace essentially all of the oil along in its travel.
Regardless of whether the carbon dioxide removes the oil by first contact miscibility or the development of the transition zone of partly dissolved reservoir oil, the frontal instability which arises from the viscosity differences between the carbon dioxide and the reservoir oil disrupt the displacement patterns in the reservoir substantially.
In general, the methods used or proposed for the control of frontal instability entail the increase of the flowing pressure gradient behind the front, that is, a decrease in the displacing fluid's mobility. An initial proposal to decrease the effective mobility of the displacing fluid, so as to increase the pressure gradient in the region it occupied was to add water to the injection fluid in the process known as WAG, or water alternated with gas. Although this procedure has been adopted in a number of applications, there are problems with its effectiveness. First, the injected water may prevent the oil forming good contact with the displacement fluid and second, early breakthrough of the displacement fluid has apparently been a common occurrence in flood programs incorporating WAG. A second proposal has been to use a foam-like dispersion of dense-phase carbon dioxide in a surfactant solution since such composite fluids would have a decreased mobility through porous rock, apparently as a consequence of the formation and migration of the aqueous films in which most of the water present in the foam is transported. These films would separate the carbon dioxide into cells, preventing it from moving freely through the pore space.
A third method for controlling frontal instability has also been proposed. In this method, a polymer is dissolved in the dense-phase carbon dioxide to form a more viscous phase having a viscosity high enough for mobility control. Such a thickened carbon dioxide fluid would be especially advantageous in several ways because no added water would be required in its use. On a microscopic scale, the absence of added water would decrease the amount of oil which is by-passed or prevented from coming into contact with the displacing fluid and in addition, corrosion problems associated with the presence of water in the injected fluid would be obviated. The principal objection to this proposal, however, is that dense-phase carbon dioxide, although a solvent, is not a very powerful one and few, readily available polymers are soluble in carbon dioxide to any substantial extent. This is a serious problem in oil field operations because not only must the material have the desired chemical and physical characteristics but they must also be relatively cheap for the operation to be economically justified. The problem of limited solubility is exacerbated, moreover, by the fact that dissolved polymer will tend to come out of solution and be deposited in the lower pressure regions of the reservoir when these regions are reached by the thickened carbon dioxide.